As is well known, tennis is played on a court marked with lines. When a ball bounces on or close to certain of the lines an umpire must rule on whether the ball is within a designated area bounded by the lines, and hence in-play ("in") or is out-of-play ("out"). Because the tennis ball may be travelling at high speed, it is often difficult to judge by eye whether the ball is "in" or "out". When tennis is being played professionally the umpires rulings may have considerable importance for players and/or sponsors.
A number of systems have been proposed for automatically detecting whether a tennis ball is "in" or "out". Most such systems utilize a tennis ball which is provided with a conductive outer surface. A plurality of closely spaced parallel exposed electrical conductors extend on and/or adjacent the lines and along the full length thereof. Contact of the conductive outer surface of the ball with adjacent conductors completes an electrical circuit. If the conductors of the circuit completed by the ball are within an "in" area of the court the apparatus signals the ball is "in". Such systems are exemplified in U.S. Pat. Nos. 3,883,860 and 1,370,333.
Tennis balls having an electrical conductive exterior for use in those systems are described in U.S. Pat. Nos. 1,580,360, 4,299,384, 4,299,029, 4,071,242, and 3,854,719.
Systems dependant on conductive connection between exposed conductors on the court surface are susceptible to failure as a result of resistive corrosion either of the conductors of the court or of the ball, or covering of the conductors by insulators such as dirt and to failure as a result of short circuits for example by moisture. Moreover the balls do not behave as do normal tennis balls or cause undue wear of racquets or the conductive surface of the ball fails as a result of wear prematurely in the ball life.
In U.S. Pat. No. 3,774,194 there is described a system which does not require exposed conductors. Instead a receiving antenna wire extends longitudinally of a court line and is buried beneath the line. There is provided a radio transmitter and a ball containing three coils at right angles acting as a resonant circuit tuned to the radiofrequency of the transmitter. The ball when in the vicinity of the court antenna acts as a coupler causing vertically polarised radiowaves from the transmitter to be sensed in the horizontal court antennae. In another embodiment a ball having a ferromagnetic metal or metal oxide included in the rubber composition thereof disturbs the resonant tuning of transmitter and receiver or the capacitance of a thin layer of metal deposited on the outer surface of the rubber ball beneath a felt outerlayer is used to unbalance a balanced capacitance bridge circuit.
That system is subject to interference by external signals and the balls required for use in the system do not have the properties of normal tennis balls and are expensive to manufacture.
None of the systems so far proposed has won wide acceptance and there is a continuing need for a satisfactory ball for use in detection systems.
An object of the present invention is to provide a ball which avoids at least some of the previously discussed disadvantages.